The present invention relates in general to electronic devices and, more particularly, to switching regulators used in power supplies.
Most switching power supplies have a transformer with a power switching transistor coupled to one side of the transformers primary winding. The power transistor turns on and off as determined by a regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops a DC output voltage across a shunt capacitor coupled across the secondary winding as a function of the energy transfer.
One type of switching power supply, a flyback power supply, can be operated in a continuous conduction mode (CCM) or discontinuous conduction mode (DCM). DCM involves switching the power switching transistor to reenergize the primary winding only after it is completely demagnetized, whereas CCM involves switching the power switching transistor even though the primary winding is still magnetized. It is preferable to operate a flyback power supply in DCM because CCM is more difficult to stabilize and turn-on losses can be significantly higher.
To operate a flyback power supply in DCM it is necessary to detect when the transformer core of the primary winding is demagnetized. At a point in time tO, the transformer core is demagnetized, which corresponds to the point when the primary current IP reaches zero. By indirectly measuring a voltage across an auxiliary winding of the flyback power supply the point when the primary current IP reaches zero can be determined. However, the auxiliary winding represents an additional element that increases manufacturing cost. Elimination of the auxiliary winding used to detect demagnetization is beneficial to reduce manufacturing cost of switching power supplies.
Accordingly, a flyback power supply that detects when the transformer core of the primary winding is demagnetized without using an auxiliary winding is needed in the art. The invention disclosed herein will address the above problems.